If you want to swim fast in Roblox Fisch, there's no better item than the Super Flippers. I picked this item up around level 20, but you could do it earlier. It's not an easy item to find without a guide, so I'm here to show you exactly how and where to get Super Flippers.
Before you get started, make sure to pick up Basic Diving Gear from Moosewood. You can purchase this for 3000 Credits on the ground next to the Shipwright by the docks. You will also need 30,000 Credits to purchase the Super Flippers. Once you have the Basic Diving Gear and enough money, you are ready to get started.
Step 1: Sail to Sunstone Island
Talk to the Shipwright and spawn your ship. Hop in and start sailing northeast to Sunstone Island. When you get to Sunstone Island, head around to the right side of the island and you'll see a tiny island with a palm tree. We'll use this as a marker.
Step 2: Find the buoy
Head directly north from the palm tree in your ship until you reach the buoy. It might take a little while until you get there, depending on the speed of your ship.
Step 3: Dive down to the cave
Equip your Basic Diving Gear, and dive down to the cave. It might get very dark, so make sure to open your in-game menu and turn on Higher Brightness and Higher Saturation.
Continue until you reach the bottom of the cave and look for the sign that says "Stop!" Make a left at the sign and continue swimming through the passageway. Follow the passageway to the end, and then swim up and you will see a Merchant. The Super Flippers are on the ground to the left of the merchant.
